P-type Doping Effect Research Articles

Abnormal high-temperature luminescence enhancement observed in monolayer MoS2 flakes: thermo-driven transition from negatively charged trions to neutral excitons

Abnormal high-temperature luminescence enhancement is observed in monolayer MoS2, owing to the joint effect of trion self-dissociation and p-type doping.

Effect of the nand p-type Si(100) substrates with a SiC buffer layer on the growth mechanism and structure of epitaxial layers of semipolar AlN and GaN

A new effect of the n-and p-type doping of the Si(100) substrate with a SiC film on the growth mechanism and structure of AlN and GaN epitaxial layers has been revealed. It has been experimentally shown that the mechanism of AlN and GaN layer growth on the surface of a SiC layer synthesized by substituting atoms on n- and p-Si substrates is fundamentally different. It has been found that semipolar AlN and GaN layers on the SiC/Si(100) surface grow in the epitaxial and polycrystalline structures on p-Si and n-Si substrates, respectively. A new method for synthesizing epitaxial semipolar AlN and GaN layers by chloride–hydride epitaxy on silicon substrates has been proposed.

Origin of Enhanced Hole Injection in Organic Light-Emitting Diodes with an Electron-Acceptor Doping Layer: p-Type Doping or Interfacial Diffusion?

The electrical doping nature of a strong electron acceptor, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN), is investigated by doping it in a typical hole-transport material, N,N'-bis(naphthalen-1-yl)-N,N'-diphenylbenzidine (NPB). A better device performance of organic light-emitting diodes (OLEDs) was achieved by doping NPB with HATCN. The improved performance could, in principle, arise from a p-type doping effect in the codeposited thin films. However, physical characteristics evaluations including UV-vis absorption, Fourier transform infrared absorption, and X-ray photoelectron spectroscopy demonstrated that there was no obvious evidence of charge transfer in the NPB:HATCN composite. The performance improvement in NPB:HATCN-based OLEDs is mainly attributed to an interfacial modification effect owing to the diffusion of HATCN small molecules. The interfacial diffusion effect of the HATCN molecules was verified by the in situ ultraviolet photoelectron spectroscopy evaluations.

Electrical and optical properties of layer-stacked graphene transparent electrodes using self-supporting transfer method

We present a systemic investigation on electrical and optical properties of layer-stacked graphene transparent electrodes using a self-supporting method (SSM). As is expected, SSM-stacked films appear smoother surface morphologies (roughness of 4.60nm for 5-layer sample) than the traditional layer stacking method. The non-contact electrical measurement demonstrates that sheet resistances of layer-stacked samples obviously diminish after the layer stacking process, which is only 20.16Ω/□ for the 5-layer-SSM-stacked sample. All the SSM samples show high white light transparency (∼88% for 5-layer sample). The ultraviolet–visible–infrared absorption spectra indicate that there are two main absorption peaks, which are related to the inter-band emission with resonant exciton effect and intra-band absorption decoupled stacked multilayer graphene, respectively. And after nitric acid doping, these peaks show obvious red-shifts because of p-type doping and coupling effects arising from the layer-stacking process. All these results reveal that there are both strong coupled and decoupled multilayer graphene in the SSM-stacked samples; and the coupling effect can be enhanced after the acid treatment.

The chemical origin of the p-type and n-type doping effects in the hybrid methylammonium-lead iodide (MAPbI3) perovskite solar cells.

In the present Communication we discuss a field-induced and photoinduced self-doping chemistry resulting in the formation of the positively and negatively charged vacancies in the MAPbI3 perovskite films. These vacancies induce p-type and n-type doping of the perovskite absorber leading to the realization of the p-i-n device operation mechanism.

Highly sensitive detection of mercury(II) ions with few-layer molybdenum disulfide

Two-dimensional (2D) layered transition metal dichalcogenide (TMD) materials (e.g., MoS2) have attracted considerable interest due to their atomically thin geometry and semiconducting electronic properties. With ultrahigh surface to volume ratio, the electronic properties of these atomically thin semiconductors can be readily modulated by their environment. Here we report an investigation of the effects of mercury(II) (Hg2+) ions on the electrical transport properties of few-layer molybdenum disulfide (MoS2). The interaction between Hg2+ions and few-layer MoS2 was studied by field-effect transistor measurements and photoluminescence. Due to a high binding affinity between Hg2+ ions and the sulfur sites on the surface of MoS2 layers, Hg2+ ions can strongly bind to MoS2. We show that the binding of Hg2+ can produce a p-type doping effect to reduce the electron concentration in n-type few-layer MoS2. It can thus effectively modulate the electron transport and photoluminescence properties in few-layer MoS2. By monitoring the conductance change of few-layer MoS2 in varying concentration Hg2+ solutions, we further show that few-layer MoS2 transistors can function as highly sensitive sensors for rapid electrical detection of Hg2+ ion with a detection limit of 30 pM. Open image in new window

First-principles study of chemical-edge-doping effect on transport properties of armchair-edge graphene nanoribbons

We have investigated the electronic transport of graphene nanoribbons (GNRs) with armchair edges containing boron, nitrogen, or oxygen, using the first-principles density functional theory. We found that such dopants at edge sites provided the n- or p-type doping effect for armchair graphene nanoribbons (AGNRs). As a result, even under an applied bias potential lower than the band gap energy of pristine AGNR, the nitrogen edge-doped GNRs show a large conductance that is almost the same as that of the ideal zigzag GNRs.

Simplified phosphorescent organic light-emitting diodes with a WO3-doped wide bandgap organic charge transport layer

The effects of p-type doping of wide bandgap ambipolar 4,4′-N,N′-dicarbazolebiphenyl (CBP) with WO3 were investigated through detailed electrical device characterization. It was found that, to achieve effective doping for improved hole injection and transport, the doping level should be greater than 20mol% and the doped layer should be at least 10nm thick. A large downward shift of the Fermi level in WO3-doped CBP causes band bending and depletion at the doped/undoped CBP interface, resulting in an additional energy barrier which hampers hole transport. Simplified green phosphorescent organic light-emitting diodes (PhOLEDs) with CBP as the hole transport and host material were fabricated. With a WO3-doped hole transport layer, the PhOLEDs attained brightness of 11,163cd/m2 at 20mA/cm2, and exhibited an improved reliability under constant-current stressing as compared to undoped PhOLEDs.

F4-TCNQ concentration dependence of the current—voltage characteristics in the Au/P3HT:PCBM:F4-TCNQ/n-Si (MPS) Schottky barrier diode

In this study, we investigate some main electrical parameters of the gold/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester:2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane/n-type silicon (Au/P3HT:PCBM:F4-TCNQ/n-Si) metal—polymer—semiconductor (MPS) Schottky barrier diode (SBD) in terms of the effects of F4-TCNQ concentration (0%, 1%, and 2%). F4-TCNQ-doped P3HT:PCBM is fabricated to figure out the p-type doping effect on the device performance. The main electrical parameters, such as ideality factor (n), barrier height (ΦB0), series resistance (Rs), shunt resistance (Rsh), and density of interface states (Nss) are determined from the forward and reverse bias current—voltage (I–V) characteristics in the dark and at room temperature. The values of n, Rs, ΦB0, and Nss are significantly reduced by using the 1% F4-TCNQ doping in P3HT:PCBM:F4-TCNQ organic blend layer, additionally, the carrier mobility and current are increased by the soft (1%) doping. The most ideal values of electrical parameters are obtained for 1% F4-TCNQ used diode. On the other hand, the carrier mobility and current for the hard doping (2%) become far away from the ideal diode values due to the unbalanced generation of holes/electrons and doping-induced disproportion when compared with 1% F4-TCNQ doping. These results show that the electrical properties of MPS SBDs strongly depend on the F4-TCNQ doping and doping concentration of interfacial P3HT:PCBM:F4-TCNQ organic layer. Moreover, the soft F4-TCNQ doping concentration (1%) in P3HT:PCBM:F4-TCNQ organic layer significantly improves the electrical characteristics of the Au/P3HT:PCBM:F4-TCNQ/n-Si (MPS) SBDs which enables the fabricating of high-quality electronic and optoelectronic devices.

Facile and surfactant-free synthesis of SnO2-graphene hybrids as high performance anode for lithium-ion batteries

A facile microwave-assisted ethylene glycol method is developed to synthesize the SnO2 nanoparticles dispersed on or encapsulated in reduced graphene oxide (SnO2-rGO) hybrids. The morphology, structure, and composition of SnO2-rGO are investigated by scanning electron microscopy, transmission electron microscope, thermo-gravimetric analyzer, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. The electrochemical performance of SnO2-rGO as anode materials for lithium-ion batteries was tested by cyclic voltammetry, galvanostatic charge–discharge cycling, and rate capability test. It is found that the SnO2 nanoparticles with a uniform distribution have p-type doping effect with rGO nanosheets. The as-prepared SnO2-rGO hybrids exhibit remarkable lithium storage capacity and cycling stability, and the possible mechanism involved is also discussed. Their capacity is 1222 mAhg−1 in the first cycle and maintains at 700 mAhg−1 after 100 cycles. This good performance can be mainly attributed to the unique nanostructure, good structure stability, more space for volume expansion of SnO2, and mass transfer of Li+ during cycling.

Two-dimensional electron and hole gases in InxGa1− xN/AlyGa1− yN/GaN heterostructure for enhancement mode operation

In this paper, a numerical study of InxGa1−xN/AlyGa1−yN/GaN heterostructure is presented. The dependence of two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) sheet densities on variables, such as InxGa1−xN layer thickness and In content, and AlyGa1−yN barrier layer thickness and Al content, are systematically investigated. The effect of P-type doping in InxGa1−xN on 2DEG and 2DHG sheet densities in this heterostructure is also studied. It is shown that the strong reverse electric field in InxGa1−xN cap layer contributes to the depletion of 2DEG at the AlyGa1-yN/GaN interface. When InxGa1−xN layer thickness and In content increases, 2DEG sheet density decreases significantly. P-type doping shows less influence on 2DEG compared to the polarization electric field in InxGa1−xN layer. In addition, there exist critical values for all the variables beyond which 2DHG appears at the interface of InxGa1−xN/AlyGa1−yN. Once 2DHG appears, it will prevent 2DEG from being further depleted. With proper design of AlyGa1−yN layer, the coexistence of 2DEG and 2DHG in InxGa1−xN/AlyGa1−yN/GaN structure can be avoided, showing that this structure has great potential in the fabrication of enhancement mode (E-mode) high electron mobility transistors.

Wavelength-versatile graphene-gold film saturable absorber mirror for ultra-broadband mode-locking of bulk lasers.

An ultra-broadband graphene-gold film saturable absorber mirror (GG-SAM) with a spectral coverage exceeding 1300 nm is experimentally demonstrated for mode-locking of bulk solid-state lasers. Owing to the p-type doping effect caused by graphene-gold film interaction, the graphene on gold-film substrate shows a remarkably lower light absorption relative to pristine graphene, which is very helpful to achieve continuous-wave mode-locking in low-gain bulk lasers. Using the GG-SAM sample, stable mode-locking is realized in a Yb:YCOB bulk laser near 1 μm, a Tm:CLNGG bulk laser near 2 μm and a Cr:ZnSe bulk laser near 2.4 μm. The saturable absorption is characterised at an intermediate wavelength of 1.56 μm by pump-probe measurements. The as-fabricated GG-SAM with ultra-broad bandwidth, ultrafast recovery time, low absorption, and low cost has great potential as a universal saturable absorber mirror for mode-locking of various bulk lasers with unprecedented spectral coverage.

Combined effects of hydrogen annealing on morphological, electrical and structural properties of graphene/r-sapphire

We proposed a comprehensive study on the combined effects of hydrogen thermal annealing process on the morphological, structural and electrical properties of graphene transferred on r-plane sapphire (1–102). We found that although thermal annealing can remove the polymeric residues, unintentional p-type doping of polymeric residues and structural breakages arise simultaneously with the in-plane tensile strain. Also, the forming mechanism of cracks and p-type doping effect were investigated through Raman spectroscopy, atomic force microscopy, non-contact Hall and high resolution X-ray photoelectron spectroscopy. It revealed thermal annealing process has a combined influence on the performance of the hydrogen annealed samples with a critical point, which was ∼250°C in this work. When the annealing temperature was over the critical point, significant p-type doping effects were included in a dominant position because of graphene with in-plane strain. The results can be important for the fabrication or chemical processing of graphene-based materials and devices.

Reducing charge trapping in PbS colloidal quantum dot solids

Understanding and improving charge transport in colloidal quantum dot solids is crucial for the development of efficient solar cells based on these materials. In this paper, we report high performance field-effect transistors based on lead-sulfide colloidal quantum dots (PbS CQDs) crosslinked with 3-mercaptopropionic acid (MPA). Electron mobility up to 0.03 cm2/Vs and on/off ratio above 105 was measured; the later value is the highest in the literature for CQD Field effect transistors with silicon-oxide gating. This was achieved by using high quality material and preventing trap generation during fabrication and measurement. We show that air exposure has a reversible p-type doping effect on the devices, and that intrinsically MPA is an n-type dopant for PbS CQDs.

Edge Oxidation Effect of Chemical-Vapor-Deposition-Grown Graphene Nanoconstriction

The edge oxidation effects of chemical-vapor-deposition-grown graphene devices with nanoconstrictions of different sizes are presented. The effects of edge oxidation on the doping level of a nanoconstriction graphene device were identified by Raman spectroscopy and using the back-gate-voltage-dependent resistance. Strong p-type doping was observed as the size of nanoconstriction decreased. The Dirac point of the graphene device shifted toward positive voltage, and the positions of the G and 2D peaks in Raman spectroscopy shifted toward a higher wave number, indicating the p-type doping effect of the graphene device. p-type doping was lifted by deep-ultraviolet light illumination under a nitrogen atmosphere at room temperature. p-type doping was restored by deep-ultraviolet light illumination under an oxygen atmosphere at room temperature. Edge oxidation in the narrow structures explains the origin of the p-type doping effect widely observed in graphene nanodevices.

Preferential Positioning of Dopants and Co-Dopants in Embedded and Freestanding Si Nanocrystals

In this work we aim at understanding the effect of n- and p-type substitutional doping in the case of matrix-embedded and freestanding Si nanocrystals. By means of ab initio calculations we identify the preferential positioning of the dopants and its effect on the structural properties with respect to the undoped case. Subsequently, we consider the case of phosphorus and boron co-doped nanocrystals showing that, against the single-doping situation, the energetics strongly favors the binding of the impurities at the nanocrystal surface. Indeed we demonstrate that the polar B-P bond forms a stable permanent electric dipole that radially points inward in the nanocrystal. Such a noteworthy characteristic and its physical consequences are discussed alongside new potential applications.

Improving chemical vapor deposition graphene conductivity using molybdenum trioxide: An in-situ field effect transistor study

By using in situ field effect transistor characterization integrated with molecular beam epitaxy technique, we demonstrate the strong surface transfer p-type doping effect of single layer chemical vapor deposition (CVD) graphene, through the surface functionalization of molybdenum trioxide (MoO3) layer. After doping, both the hole and electron mobility of CVD graphene are nearly retained, resulting in significant enhancement of graphene conductivity. With coating of 10 nm MoO3, the conductivity of CVD graphene can be increased by about 7 times, showing promising application for graphene based electronics and transparent, conducting, and flexible electrodes.

Electronic structure of MoO3−x/graphene interface

The interfacial electronic structure of MoO3−x/graphene has been investigated using photoemission spectroscopy. The experimental data showed that upon deposition of MoO3−x, the Fermi level of graphene shifts downward gradually from its Dirac point due to the p-type doping effect. From the Fermi level shift of −0.28eV, the hole density of graphene was estimated to be 5.44×1012cm−2. The formation of surface negative dipole due to electron transfer from graphene to the deposited MoO3−x films increased the sample′s work function. The existence of gap states in MoO3−x induced by oxygen vacancies greatly reduced the hole injection barrier at the MoO3−x/graphene interface.

Dual functions of YF3:Eu3+ for improving photovoltaic performance of dye-sensitized solar cells

In order to enhance the photovoltaic performance of dye-sensitized solar cell (DSSC), a novel design is demonstrated by introducing rare-earth compound europium ion doped yttrium fluoride (YF3:Eu3+) in TiO2 film in the DSSC. As a conversion luminescence medium, YF3:Eu3+ transfers ultraviolet light to visible light via down-conversion, and increases incident harvest and photocurrent of DSSC. As a p-type dopant, Eu3+ elevates the Fermi level of TiO2 film and thus heightens photovoltage of the DSSC. The conversion luminescence and p-type doping effect are demonstrated by photoluminescence spectra and Mott-Schottky plots. When the ratio of YF3:Eu3+/TiO2 in the doping layer is optimized as 5 wt.%, the light-to-electric energy conversion efficiency of the DSSC reaches 7.74%, which is increased by 32% compared to that of the DSSC without YF3:Eu3+ doping. Double functions of doped rare-earth compound provide a new route for enhancing the photovoltaic performance of solar cells.

Enhanced performance of inverted organic solar cell by a solution-based fluorinated acceptor doped P3HT:PCBM layer

An inverted organic solar cell based on strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped poly (3-hexylthiophene) P3HT: [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was fabricated to figure out the p-type doping effect on the device performance. It was found that the doping concentration played a critical role on the electrical output of the devices. An enhanced power conversion efficiency (PCE) of 4.22% was achieved, in comparison of PCE of 3.68% for the device based on pristine P3HT:PCBM. The topography morphology of the active film, the hole mobility, the ultraviolet–visible absorption spectrum, the photoluminescence (PL) lifetime and the Fermi energy level of P3HT film with and without F4-TCNQ doping were characterized to investigate the doping effect. The measured results indicated that the hole mobility and absorption of P3HT film was slightly modified with F4-TCNQ doping. On the contrary, the active film morphology, the PL lifetime and the Fermi energy level of P3HT changed dramatically with doping. It was found that F4-TCNQ preferred to approach to the air/liquid interface during the solvent dry process, leading to F4-TCNQ-rich upper layer due to its low surface energy. The layer with proper thickness between anode and active layer dramatically improve the interface contact, resulting in the enhanced device performance.